Most cities and counties have enacted requirements for industries which discharge water into the municipal sewer systems. These municipalities generally require the waste water to meet certain standards before being discharged into the sewer systems, and these standards usually relate to the level of pollutants which interfere with the efficient operation of the municipal waste water treatment facility.
Waste water which contains materials having a high biochemical oxygen demand, total suspended solids and ammonia increase the treatment cost to the city or county. For purposes of this invention, the term "biochemical oxygen demand" ("BOD") refers to the quantity of oxygen utilized in the biochemical oxidation of organic matter; and the term "total suspended solids" ("TSS") refers to the total suspended solids which float on the surface of, or are suspended in, waste water and which are removable by filtering.
In some instances, a surcharge may be levied on an industrial plant that produces waste water which does not meet the standards as established by the municipality. For example, these standards may prohibit waste water containing greater than 300 mg/l of BOD, 300 mg/l of TSS, and/or 18 mg/l of ammonia. Cities and counties may also prohibit industrial plants from discharging waste water having an oil and grease concentration greater than 150 mg/l.
A city or county also has the authority to revoke an industrial plant's waste water discharge permit if that plant continues to discharge waste water which does not meet the standards set by that city or county. Therefore, an industrial plant must effectively treat its waste water to reduce the level of pollutants and thereby meet such standards.
Furthermore, an effective waste water treatment process should cause the solid organic and inorganic matter to flocculate and form a sludge cake. This sludge cake accumulates to a certain thickness and is then separated from the liquid component of the waste water and sent to a rendering plant for further processing. If the resulting sludge cake is under 18% solids, the rendering plant may impose a surcharge because sludge with a high percentage of water is more expensive for the rendering plant to process.
One of the most common methods of treating waste water is the dissolved air flotation method ("DAF") which injects dissolved air into the waste water, causing the solids to rise to the top. However, the DAF method tends to produce a sludge cake with a low percentage of solids (typically, less than 10%).
Another method of treating waste water is to use a clarifying or settling tank with a filter at the bottom. The waste water is chemically treated within the tank, and the solids sink to the bottom of the tank. The liquid component of the waste water is directed out of the tank, and a wet cake is left on the filter. This method produces a sludge cake with typically, less than 10% solids.
With the DAF method or the settling tank method, a belt press can be used to produce a sludge cake with a 20%-30% solids content. However, this step requires additional equipment and expense.
Therefore, there is a long felt need for an economically feasible method that effectively treats (a) industrial waste water to meet the standards as set by a city or county so that such waste water can be directed into a municipal sewer system and (b) produces a sludge cake with a high percentage of solids.